1. Field of the Invention
Exemplary embodiments of the present patent application relate to a sheet feeding device, an electrophotographic image forming apparatus incorporating the sheet feeding device, and a sheet separation method for the image forming apparatus, and more particularly, to a sheet feeding device that electrostatically attracts a sheet of a recording medium to an endless belt for separating and feeding the sheet therefrom, an electrophotographic image forming apparatus incorporating the sheet feeding device, and a sheet separation method for the image forming apparatus.
2. Discussion of the Related Art
Related-art image forming apparatuses, such as electrophotographic copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a sheet of recording media according to image data. Thus, for example, a sheet feeding device loads a plurality of sheets and feeds them one by one toward an image forming device. The image forming device forms an image on a sheet supplied from the sheet feeding device.
Sheet feeding devices generally employ an air suction method in which negative pressure is generated by air suction to suction and transport recording media, an electrostatic sheet feed method in which recording media are electrically attracted to a sheet separation member and separated from a stack of recording media one by one, or the like.
As one approach, the sheet feeding device employing the air suction method includes an endless belt formed into a loop having multiple air suction holes formed therein and an air suction duct disposed inside the loop of the endless belt. As a suction force is generated by the air suction duct, an uppermost sheet of a stack of sheets placed on a sheet loading table is suctioned and attracted to the surface of the endless belt.
Generally, a sheet lifting and separating unit is provided together with the suction-type sheet feeding device. The sheet lifting and separating unit sends air to the leading edge of several upper sheets of a stack of sheets placed on the sheet loading table, lifting up the several upper sheets including the uppermost sheet while enabling the sheet lifting and separating unit to separate the uppermost sheet from the other sheets in the stack.
As another approach, the electrostatic sheet feed method is employed in a sheet feeding device that can be incorporated in an electrostatic image forming apparatus. The sheet feeding device includes an endless dielectric belt and a charging member for electrically charging and discharging (more precisely, removing the charge from) the surface of the endless dielectric belt.
The endless dielectric belt that rotates in a sheet feeding direction contacts a top surface of a stack of sheets, and the charging member applies alternating charges (that is, electrical charges of alternating polarity) to the surface of the endless dielectric belt. The charging member performs both a charging operation to form an alternating charge pattern on the surface of the endless dielectric belt and a discharging operation to discharge the surface of the endless dielectric belt.
With the above-described configuration, the electrostatic sheet feeding device charges the surface of the endless dielectric belt to form an electric field that generates a force of attraction to separate the uppermost sheet from the other sheets of the stack of sheets.
However, when separating sheets under high-humidity conditions or with cutting burrs at the edges thereof, the above-described sheet feeding devices cannot reliably separate the uppermost sheet from any subsequent sheets of the stack of sheets, causing multiple-sheet feeding.
To avoid such multiple-sheet feeding, Japanese Patent Application Publication No. 2001-097580 (JP-2001-097580-A) describes a sheet feeding device employing the air suction method that includes a paper warping means having a pair of paper engaging members, a pivoting member, a cam, and a cam shaft. The pair of paper engaging members engages the upper surface of a top sheet of paper across a predetermined gap in a width direction of the sheet stack. One end of the pivoting member is connected to the pair of engaging means. The pivoting member rotates about a supporting bracket provided at the upper part of the pair of paper engaging members. The cam engages the other end of the pivoting member and is rotatably fixed to the cam shaft. The paper warping means causes the pair of paper engaging members to closely contact to or separate from the sheet stack in the width direction, via the pivoting member as the cam shaft rotates.
The sheet feeding device disclosed in JP-2001-097580-A performs a sub-separation operation prior to a sheet separation operation performed by a sheet lifting and separating unit. In the sub-separation operation, the cam shaft is rotated to cause the pair of paper engaging members to approach each other via the cam and the pivoting member in the width direction of the sheet stack. By so doing, the top sheet of paper engaged by the pair of paper engaging members warps between the pair of paper engaging members to curve upwards latitudinally. As a result, a space is formed between the uppermost sheet and any subsequent sheets. By sending air into the space using the sheet lifting and separating unit, the uppermost sheet can be separated from the subsequent sheets.
However, for performing the sub-separation operation prior to the sheet separation operation for an uppermost sheet by the sheet lifting and separating unit, the related-art sheet feeding device disclosed in JP-2001-097580-A must have a sheet warping unit with a complex configuration, which in turn complicates the configuration of the sheet feeding device and increases the cost.
Further, the related-art sheet feeding device cannot feed individual sheets sheet by sheet from a stack of sheets reliably. In the related-art sheet feeding device disclosed in JP-2001-097580-A, the top sheet of paper is attracted to an endless belt having multiple air suction holes formed therein while the top sheet of paper is warped upwardly between the pair of paper engaging members in the width direction of the sheet stack. Therefore, the top sheet of paper is not flat when it is attracted to and contacts the endless belt, meaning that the attraction is unstable. In addition, sheets of paper warp differently, which is likely that the top sheet of paper is attracted to different points on the surface of the endless belt and the sheet of paper may be skewed with respect to the proper sheet feeding direction. If the sheet of paper is fed and conveyed in this condition, the result might be deviation in sheet feeding timing, disrupting the entire image forming process.